Geology Reference
In-Depth Information
held close to the body. Compasses, pocket knives and geological hammers
are all detectable at distances of less than about a metre, and users of
high-sensitivity magnetometers may need to visit tailors (and cobblers) for
non-magnetic clothing. Survey vehicles can be detectable at distances of up
to 20 m. The safe distance should be determined before starting survey work.
Absolute magnetometers should all give the same reading when read
at the same time and in the same place. Differences between instruments
manufactured prior to 1980 were often greater than 10 nT, but now are
seldom more than 1 or 2 nT. Sensors can be placed very close together and
may even touch when checks are being made, but proton magnetometers
cannot be read exactly simultaneously in this way because the two polarising
fields would interfere.
Large discrepancies and very variable proton magnetometer readings
usually indicate poor tuning. The correct tuning range can be roughly iden-
tified using global maps (see Figure 3.3) but final checks should be made
in the field. Near-identical readings should be obtained if the setting is
varied over a range of about 10 000 nT about its optimum position (e.g.
47 000 in Example 3.1). Manual versions are generally rather coarsely tune-
able in steps of a few thousand nT, but greater accuracy is possible with
microprocessor control. It is partly this finer tuning that is now allowing
proton magnetometers to be routinely read to 0.1 nT. They may be pro-
grammed to respond to faulty tuning or high gradients by refusing to display
the digit beyond the decimal point.
Example
3.1: Proton magnetometer tuning (manual model)
Tuning setting Readings
30 000 31 077 31 013 31 118
32 000 32 770 32 788 32 775
34 000 35 055 34 762 34 844
36 000 37 481 37 786 37 305
38 000 42 952 40 973 41 810
41 000 47 151 47 158 47 159
44 000 47 160 47 158 47 159
47 000 47 171 47 169 47 169
50 000 47 168 47 175 47 173
53 000 47 169 47 169 47 169
56 000 53 552 54 602 54 432
60 000 59 036 59 292 58 886
64 000 65 517 65 517 65 517
The readings at the 64,000 nT setting show that repeatability alone is no
guarantee of correct tuning. It is the range of settings over which the circuits
can lock to the precession signal that provides the crucial evidence.
